Multiple OPTIONALs mishandled

[niklas88]

Take the following query, each OPTIONAL on its own works giving an empty column. With both OPTIONAL the result is empty. It looks like the second OPTIONAL is done as a normal Join.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX schema: <http://schema.org/>
SELECT ?qleverui_entity ?qleverui_name ?qleverui_altname ?qleverui_count WHERE {
    {
      SELECT ?qleverui_entity (COUNT(?about) AS ?qleverui_count) WHERE {
        ?about schema:about ?x .
        ?about ql:has-predicate ?qleverui_entity .
      }
      GROUP BY ?qleverui_entity
    }
    OPTIONAL {
      ?qleverui_claim <http://wikiba.se/ontology#directClaim> ?qleverui_entity .
      ?qleverui_claim @en@<http://www.w3.org/2000/01/rdf-schema#label> ?qleverui_name .
    }
    OPTIONAL {
      ?qleverui_claim_2 <http://wikiba.se/ontology#directClaim> ?qleverui_entity .
      ?qleverui_claim_2 @en@<http://www.w3.org/2004/02/skos/core#altLabel> ?qleverui_altname .
    }

}
ORDER BY DESC(?qleverui_count)
LIMIT 40

[niklas88]

Here is a reproducer on the scientists KB, this should give us a list of all Scientists, not only 134.

SELECT ?x ?height ?partner WHERE  {
        ?x <is-a> <Scientist> .
        OPTIONAL { ?x <Height> ?height . }
        OPTIONAL { ?x <Spouse_(or_domestic_partner> ?partner}
}

And this is the operations tree, I think the outer most Join should also be an OptionalJoin

│
├─ Join on ?x
│  result_size: 134 x 3
│  columns: ?x, ?height, ?partner
│  total_time: 0.00 ms
│  operation_time: 0.00 ms
│  cached: true
│    original_operation_time: 0.00
│    original_total_time: 0.00
│  ┬
│  │
│  ├─ OptionalJoin on ?x
│  │  result_size: 134 x 3
│  │  columns: ?x, ?height, ?partner
│  │  total_time: 0.00 ms
│  │  operation_time: 0.00 ms
│  │  cached: false
│  │  ┬
│  │  │
│  │  ├─ IndexScan ?x <Height> ?height
│  │  │  result_size: 134 x 2
│  │  │  columns: ?x, ?height
│  │  │  total_time: 0.00 ms
│  │  │  operation_time: 0.00 ms
│  │  │  cached: true
│  │  │    original_operation_time: 26.00
│  │  │    original_total_time: 26.00
│  │  │
│  │  ├─ IndexScan ?x <Spouse_(or_domestic_partner> ?partner
│  │  │  result_size: 0 x 2
│  │  │  columns: ?x, ?partner
│  │  │  total_time: 0.00 ms
│  │  │  operation_time: 0.00 ms
│  │  │  cached: false
│  │
│  ├─ IndexScan ?x <is-a> <Scientist>
│  │  result_size: 19,361 x 1
│  │  columns: ?x
│  │  total_time: 0.00 ms
│  │  operation_time: 0.00 ms
│  │  cached: true
│  │    original_operation_time: 38.00
│  │    original_total_time: 38.00